For movement and rotation of objects without changing the inclination of the objects, so-called SCARA-type robots are primarily used today. These robots are designed for the four degrees of freedom, x, y, z and rotation of an object around the z-axis. For manipulation of the object in the xy-plane, two series-connected arms are used that operate in the xy-plane, the axes thus being perpendicular to the xy-plane. The fact that the arms are connected in series implies that one arm supports the other arm, which in turn supports the object. To obtain a movement in the z-direction, a linear movement device is used. This device may be located either after the series-connected arms or before the series-connected arms in the serial kinematic chain of the robot. In the former case, the series-connected arms must move the drive package for the z-movement and in the latter case the drive package for the z-movement must move the series-connected arms. The drive package for rotating the object around the z-axis will always be located furthest out in the kinematic chain of the robot.
The series connection of the arms of the SCARA robot, as well as in other robots with series-connected links, implies that the robot is given a large movable mass, that the robot structure is weak, that the accuracy is limited and that large motor torques are required to make fast movements.
Several of the drawbacks that are associated with the SCARA robot are overcome by a robot that manipulates a platform with three parallel-working arms. This is referred to as a parallel kinematic structure. To obtain a rigid arm system with a large loading capacity and a low weight, the outer arms of the parallel-kinematic manipulator, nearest the manipulated platform, shall consist of six links in total, which only transfer compressive and tensile forces. A manipulator for movement of a platform in space is previously known, where the platform has the same inclination and orientation in its entire working range. The known robot has three parallel-working arms, each having its own linkage. It is known in this context that the total number of links is six and that they may be optionally distributed on the arms according to the combinations 2/2/2 or 3/2/1.
To more readily describe parallel-kinematic robots comprising linkages, some definitions of different linkages are introduced here:                Link: A link is a member that movably joins two elements and that, at each end, allows movement in three degrees of freedom. It usually consists of a rigid elongated member such as, for example, a rod that has a ball joint at each end. The link holds the elements at a definite distance from each other and absorbs only tensile or compressive forces. Thus, a link transfers no torsional movements.        Double link: A double link is a member that movably joins two elements, that at each end allows movement in three degrees of freedom, and that transfers a moment in a plane between the elements. The double link consists of a quadrangle with, for example, two links, according to the above, that form a first pair of links and the elements that form the second pair of links. In a special case, the double link is a parallelogram, in which case the two elements are forced to move in parallel with each other. Since all joints allow movement in three degrees of freedom, this implies that the double link may twisted. Thus, the double link needs help from other linkages to remain plane.        Locked double link: A locked double link is a member that movably joins two elements that, at each end, allows movement in two degrees of freedom. The locked double link consists of a double link according to the above, wherein at least one diagonal is locked. This is achieved, for example, by introducing in the quadrangle an additional link that is not parallel to any of the other links. This prevents the elements from being displaced, but still the locked double link may be twisted.        Triple link: A triple link is a member that movably joins two elements, that at each end allows movement in three degrees of freedom, and that transfers a moment in two planes between the elements. The triple link usually consists of two double links, oriented in different planes, with one common link. In a special case, the triple link comprises a space parallelogram consisting of three parallel links of equal length. Such a space parallelogram may be twisted but maintains the elements oriented in parallel planes.        Triangle: A triangle is a member that movably joins two elements and that at one end (the base) allows movement in one degree of freedom and at its other end allows movement in three degrees of freedom. The triangle consists, for example, of a torsionally rigid member that, at its base, is journalled to a first element through an axis and at its other end is journalled to a second element by means of a ball joint. A triangle may also consist of two links according to the above, where one of the joints is common.        
In the following text, an arm for a robot shall mean a linkage supported by a supporting arm. By the concept supporting arm is to be understood a torsionally rigid member that movably joins two elements together and that, at both ends, allow movement in one degree of freedom. The supporting arm consists, for example, of a tube with a fork arranged at each end through which an axle passes. In a special case, the axles are parallel whereby the elements joined by the supporting arm are allowed movement in one plane only. It should be mentioned that the movement comprises rotation as well as translation. The supporting arm may, from one element to the other, transfer both tensile and compressive forces, torsional moment and bending moment.
With the linkages defined above, the first of the known systems may be defined as a manipulator with three arms, each one consisting of a supporting arm and a double link. The second known manipulator may be defined as a manipulator with three arms wherein the first arm consists of a supporting arm and a link, the second arm of a supporting arm and a double link, and the third arm of a supporting arm and a triple link. It should be pointed out here that when designing according to the configuration 2/2/2, the axles of the supporting arms must cross each other to obtain an unambiguous orientation of the movable element.
For a fully extended parallel-kinematic robot for movement of a platform with three degrees of freedom (e.g. in directions x, y and z in a Cartesian system of coordinates), three parallel-working arms are required. If all six degrees of freedom of the platform (x, y, z and the tool orientation) are to be manipulated, six parallel-working arms are required. Each such arm comprises an upper arm and a lower arm. In several applications, a manipulation with a combination of degrees of freedom for positioning and degrees of freedom for orientation is desired. One class of such applications is interior work in narrow spaces. In that case, it is often desired to have a robot with two degrees of freedom for tool orientation and only one degree of freedom for radial positioning.
From U.S. Pat. No. 5,539,291, a parallel kinematic manipulator is previously known. A centre pillar supports a supporting arm operable around two axes. This supporting arm supports, in turn, a second supporting arm that supports a movable element. A first and a third supporting arm journalled about the same axis are connected to the movable element by means of outer arms comprising wires that, with respect to their function, may be likened to a combination of a supporting arm and a double link. The outer arms, as well as the second supporting arm, are arranged to transmit tensile and compressive forces as well as torsional moments. This entails a clumsy design of the manipulator. From the point of view of smoothness and the like, this manipulator cannot compare favourably with a corresponding manipulator where the outer arms only absorb tensile and compressive forces.
From WO98/30366, a parallel kinematic manipulator is previously known. Three arms including linkages here join together a stationary element and a movable element. Three actuators fixed to the stationary element each operate an arm. A first arm includes a supporting arm with a triple link. A second arm includes a supporting arm and a double link. A third arm includes a supporting arm and a link. The links included in these linkages only need to transmit compressive and tensile stresses, which makes them very rigid, although they are designed with small dimensions and of a light material. In addition, the joints are only subjected to a normal force from the links and the bearings may therefore be made light, stiff and accurate.
From WO99/58301, a further parallel kinematic manipulator is previously known. Also here, three arms including linkages join together a stationary element and a movable element. Three actuators fixed to the stationary element each operate an arm. All arms include a supporting arm with a double link, wherein the arms transmit compressive and tensile stresses only. This arm structure has been made possible by designing the manipulated platform as a frame structure in three dimensions instead of a platform in the form of a plane structure in two dimensions, as in the above known manipulator (WO98/30366).
Both in the case of a two-dimensional and in the case of a three-dimensional manipulated platform according to the above-referred known manipulators, the problem arises that the arm that provides the movement of the robot in the vertical direction (in the z-direction) is influenced by large forces. In the case of large movements of the other arms (in the xy-plane), this arm will be influenced not only by torsional moments but also by strong bending moments. Especially unfavourable are forces that, in the outer end of the arm, act in an axial direction. The design implies that, when the robot is extended, said arm in the vertical direction will be located obliquely between the other two arms, as well as when the robot is folded together. Because of the kinematics, said arm for the z-manipulation will be located midway between the other two arms only within a narrow radial distance from the centre of the robot. This entails the following disadvantages:                The working range is limited by the fact that the arm for z-manipulation is given an unfavourable position relative to the other arms.        The rigidity of the robot will be limited by the fact that the arm for z-manipulation places at least one articulated rod in an unfavourable direction relative to the manipulated platform.        The accuracy of the robot will be limited by the extra kinematic non-linearity created by the arm for z-manipulation.        The dynamic properties and the rapidity of the robot are limited by the fact that the arm for z-manipulation is subjected to oblique dynamic forces.        The kinematics of the robot does not automatically provide radial movement planes but a complicated mathematical compensation must be made in the control system for the curvature caused by the arm for z-manipulation.        